Mobile app install ads simply recommend apps for Facebook users to download. They're widely used by mobile game companies and tech startups — companies whose entire business revolves around apps. On Facebook, mobile app install ads have also been of interest to e-commerce and online retail companies, because if you can persuade someone to download a mobile shopping or booking app then a company can potentially generate a high level of lifetime revenue from that customer.

Moreover, Facebook recently doubled down on mobile app ads by adding a new feature, mobile app engagement and conversion ads, which basically remind people who clicked on an app ad previously to check out the app again. Facebook would not have launched that product, presumably, if app downloads were not a priority.

Here's what this means for Twitter: As 60 percent of users access Twitter on mobile devices, Twitter could greatly increase its revenues if it could successfully persuade app developers and users that Twitter is a good place to hear about new apps from friends and followers.

Twitter has increased its exposure to mobile ads and apps of late with its $350 million acquisition of MoPub, a mobile ad exchange that, eventually, will allow advertisers to use Twitter's data to target ads at people using other mobile apps.